oppress vs dun

oppress is not a noun while dun is a noun.

Word	Noun	Adjective	Verb	Adverb
oppress	No	No	Yes	No
dun	Yes	Yes	Yes	No

As verbs, dun is a hypernym of oppress; that is, dun is a word with a broader meaning than oppress:

Other hypernyms of oppress include bedevil, crucify, frustrate, rag, torment.

oppress (noun)	dun (noun)
	horse of a dull brownish grey color
	a color or pigment varying around a light grey-brown color

oppress (adjective)	dun (adjective)
	of a dull greyish brown to brownish grey color

oppress (verb)	dun (verb)
come down on or keep down by unjust use of one's authority	make a dun color
cause to suffer	cure by salting
	persistently ask for overdue payment
	treat cruelly